Hydraulic dampers of the semi-active type are known in the art: see, e.g., U.S. Pat. Nos. 3,807,678, 4,468,050, 4,468,739 and 4,491,207. Such dampers differ from purely passive ones in that the flow of hydraulic fluid between opposite ends thereof is rapidly and continuously controlled during operation of the dampers. They differ from fully-active dampers or actuators in that they do not require a pump or similar source of pressurized hydraulic fluid. When operated in accordance with an appropriate control policy, semi-active dampers are potentially capable of providing vibration attenuation greatly superior to that provided by passive dampers, and closely approaching that provided by fully-active hydraulic dampers or actuators, while being much less expensive than fully active dampers to acquire and operate.
If the benefits potentially available from a semi-active damper are to be realized in an actual vehicle suspension or other "real-world" environment, as opposed to merely in the laboratory, the damper should be of compact, durable and economical construction. Additionally, the valve components of the damper must operate reliably and rapidly even under adverse operating conditions. The use in a semi-active damper for a motorcycle of a control valve of the "voice-coil" type is disclosed in U.S. Pat. 4,491,207. Valves of such type work satisfactorily when they are of relatively small size and are not subjected to large temperature differentials. However, difficulties have been encountered when such valves are used in dampers that are of large size, and/or that are subjected to elevated temperatures. In such utilization, the movable annular member or members of the valve means may at times not move freely between its flow-permitting and flow-restricting position, and/or may permit excessive leakage when in the latter position thereof.
With the foregoing in mind, the primary object of the present invention is the provision of a semi-active damper assembly that is of durable, compact and economical construction and that has improved valve means.